Unit 29 DC Generators

Transcription

1 Objectives: Discuss the operation of DC generators. List the factors that determine the amount of output voltage produced by a generator. List the three major types of DC generators.

2 Objectives: List different types of armature windings. Describe the differences between series and shunt field windings. Discuss the operating differences between various types of generators.

3 Objectives: Discuss schematic diagrams for different types of DC generators. Show the brushes set to the neutral plane position on the commutator of a DC machine.

4 Construction A generator is a device that converts mechanical energy into electrical energy. Direct-current generators operate on the principle of magnetic induction. Loops of wire cut through magnetic flux lines when the shaft is mechanically turned.

5 Construction Brushes and sliprings are used to provide a pathway for electrical connections to external loads needing electrical power. The voltage produced in all rotating armatures is alternating voltage. A commutator is used to allow DC power generation.

6 Basic generator construction.

7 Cutaway graphic of a generator at 0.

8 Cutaway graphic of a generator at 90.

9 Cutaway graphic of a generator at 180.

10 Cutaway graphic of a generator at 270.

11 Cutaway graphic of a generator at 360.

12 The commutator is used to convert the AC voltage produced in the armature into DC voltage.

13 Commutator action at 0.

14 Commutator action at 90.

15 Commutator action at 180.

16 Commutator action at 270.

17 Commutator action at 360.

18 Increasing the number of turns increases the output voltage.

19 Increasing the number of loops produces a smoother output voltage.

20 The loops of wire are wound around slots in a metal core.

21 Construction The brushes ride against the commutator segments and are used to connect the armature to the external circuit of the DC machine.

22 Construction Pole pieces located inside the housing provide the magnetic field required for the generator action.

23 Construction The pole pieces are electromagnets that replace the permanent magnets in a simple generator. A magneto is a DC generator that uses permanent magnets as its field.

24 Construction Most DC machines use wound electromagnets to provide the magnetic field. There are two types of field windings: series field windings and shunt field windings.

25 Series and shunt field windings.

26 Construction Simple schematics show the connections of the armature to the series and shunt field windings. Generators fall into three main categories: 1. Series generators 2. Shunt generators 3. Compound generators

27 Series generator schematic.

28 Shunt field generator windings are connected in parallel with the armature.

29 Long shunt generator schematic.

30 Short shunt generator schematic.

31 Review: 1. A generator is a machine that converts mechanical energy into electrical energy. 2. Generators operate on the principle of magnetic induction. 3. Alternating current is produced in all rotating armatures.

32 Review: 4. The commutator changes the alternating current produced in the armature into direct current. 5. The brushes are used to make contact with the commutator and to carry the output current to the outside circuit.

33 Review: 6. The loops of wire, iron core, and commutator are made as one unit and are referred to as the armature. 7. The armature connection is marked A 1 and A Series field windings are made with a few turns of large wire and have a very low resistance.

34 Review: 9. Series field windings are connected in series with the armature. 10.Series field windings are marked S 1 and S Shunt field windings are connected in series with the armature.

35 Review: 12.Shunt field windings are connected in parallel with the armature. 13.The shunt field windings are marked F 1 and F Series generators increase their output voltage as load is added.

36 Review: 15.Shunt generators decrease their output voltage as load is added. 16.The voltage regulation of a DC generator is proportional to the resistance of the armature. 17.Compound generators contain both series and shunt field windings.

37 Review: 18.A long shunt compound generator has the shunt field connected in parallel with both the armature and the series field. 19.A short shunt compound generator has the shunt field connected in parallel with the armature, but in series with the series field.

38 Review: 20.Interpoles are small pole pieces connected between the main field poles and are used to help correct armature reaction. 21.Interpoles are connected in series with the armature.